New scion-rootstock combinations for diversification of sweet orange orchards in tropical hardsetting soils

Naturally occurring subsoil horizon restricting root development and a narrow genetic base of scion-rootstock combinations make the citriculture in Brazilian Northeast prone to biotic and abiotic stresses. The diversification of citrus orchards through the introduction of new scion-rootstocks combinations is an important strategy to reduce the risks associated with these constraints. Three experiments aiming at identifying appropriate scion-rootstock combinations for enhancing genetic diversification in citrus orchards under rainfed conditions in tropical hardsetting soils were established in Sergipe state in 2008. In each one, a different sweet orange ('Sincora', 'Valencia Tuxpan' and 'Pineapple') was grafted on six rootstocks ('Santa Cruz' Rangpur lime, 'Red Rough' lemon, 'Orlando' tangelo, 'Sunki Tropical' mandarin, HTR-051 and VKLxRPL-010 hybrids). After eight years the experiments were evaluated for plant development, yield performance and fruit quality. In general, all sweet oranges grafted on 'Red Rough' lemon showed great development and cumulative yield as does with 'Santa Cruz' Rangpur lime, with the later showing better fruit quality as an advantage. Inversely, HTR-051 hybrid displayed low yields despite high yield efficiency, for this rootstock induces dwarfism. 'Sunki Tropical' mandarin brought high yields of medium quality fruits, while the VKLxRPL-010 hybrid induced productive precocity, especially for 'Sincora' sweet oranges. Based upon these results, all tested scions grafted on 'Red Rough' lemon and 'Santa Cruz' Rangpur lime, followed by 'Sunki Tropical' mandarin are indicated for genetic diversification of groves, when planted at conventional density. On the other hand, the hybrid HTR-051 seems to have great potential for high density orchards, since it shows high yield efficiency and good quality of fruit.